Re: IPSEC: tunnel breakage with out-of-order IPv4 fragments

[Karl Heiss <kheiss@gmail.com>]

On Wed, Jul 16, 2014 at 6:59 AM, Steffen Klassert
<steffen.klassert@secunet.com> wrote:
> On Tue, Jul 15, 2014 at 08:13:01AM -0400, Karl Heiss wrote:
>> On Tue, Jul 15, 2014 at 5:16 AM, Steffen Klassert
>> <steffen.klassert@secunet.com> wrote:
>> > On Mon, Jul 14, 2014 at 07:52:23AM -0400, Karl Heiss wrote:
>> >> On Mon, Jul 14, 2014 at 5:33 AM, Steffen Klassert
>> >> <steffen.klassert@secunet.com> wrote:
>> >> >
>> >> > Your tcpdump looks interesting. Is it possible that all your
>> >> > fragmented packets have the id field set to 'id 0'? This should
>> >> > be only the case if the DF flag is set on that packet, but this
>> >> > is apparently not the case here. If all the fragmented packets
>> >> > have id 0, it is not possible to determine the correct fragment
>> >> > chain. If only one fragment gets lost, all further packets might
>> >> > be reassembled wrong.
>> >> >
>> >>
>> >> Yes, all fragments have 'id 0'.
>> >>
>> >> > When looking at the code, is seems that sctp sets the DF flag
>> >> > on packets as the default. The IPsec encapsulation code copies
>> >> > the DF bit from the inner header and sets 'id 0' in this case.
>> >> > A first guess would be that someone removes the DF flag after
>> >> > the IPsec encapsulation.
>> >> >
>> >> > Is the DF flag set on your inner sctp packets?
>> >> >
>> >>
>> >> Yes, the inner packets have DF set, but the outer do not.
>> >
>> > So we need to find where the DF flag disappears.
>>
>> I feel like we may be focusing on two different things.  I am more
>> interested in figuring out why the receive side does not handle these
>> packets gracefully.  I would assume that the missing/reordered
>> fragments may not get reassembled correctly and would be dropped,
>> which is OK.  However, it is when this event occurs and then every
>> subsequent, correctly ordered, fragmented packet is dropped that I am
>> concerned about.  While the sender may be in a broken state, the
>> receiver should be consistent with receive behavior, agreed?
>
> Ugh. No, not at all. The sender causes these problems on the receive
> side by using 'id 0' on all fragments. The id field is used to
> determine which fragments belong to which packet. The id must
> be unique for each fragmented packet. I.e. all fragments of a
> given packet must have the same id, fragments of other packets
> must have different id values. If all fragmented packets have
> the same id, they get reassembled in the order they arrive.
> Say the second fragment of packet A gets lost, then the first
> fragment of Packet A is reassembled with the second fragment
> of packet B and so on. This leads to authentication failures
> as you observe it.

If this is the case, is that not a security concern?  Anyone can spoof
fragments with just the source and destination IPs, id of 0, the SPI,
and cause any subsequent fragments to be invalidated, regardless of
order.  You say that fragments get reassembled in the order that they
arrive, but the code says otherwise, since it pays attention to the MF
and offset values.  I 100% agree that the receiver cannot possibly
differentiate between fragments when all of them have 'id 0', but it
should be able to recover once the reordering event has passed due to
the MF and offset values.  Should the receive side not be smart enough
to at least drop packets that do not authenticate due to reordering
but subsequent packets which are ordered correctly pass through
cleanly?

>
>>
>> >
>> > Can you describe your usecase more precisely? Do you use
>> > any additional tunnel like ipip/gre etc. or packet mangling?
>>
>> I apologize, I did leave out one critical bit of information in that
>> the sender is based on a RHEL 6.5 kernel with a backported 3.4.75 SCTP
>> stack.  As for other mangling or anything else, the case is as
>> straightforward as originally described.  I will try and see if I can
>> find which combination of commits need to be removed to allow this
>> case on the sending side.  I didn't think to elaborate on the sending
>> side as I was solely concentrating on the receive aspect :(
>
> Please try with unpatched kernels from kernel.org on the sender and
> the receiver.